As originally described, a jet spouted fluidised bed dryer consists essentially of a vertical chamber in which the lower part is conical with its narrow end at the bottom, and in which the upper part is a cylinder having essentially the same diameter as the upper wide end of the lower conical part. The top end of the cylinder is closed by an essentially flat horizontal plate which includes a gas outlet. The chamber also contains a suitable quantity of inert carrier particles. A hot high velocity gas stream is injected into the dryer at the lower end of the conical lower part through a gas inlet port, which serves to fluidize and to spout the carrier particles. At the junction of the gas port and the lower end of the conical part a suitable screen is provided to prevent the carrier particles from entering the gas port. The heated carrier particles are initially propelled by the hot gas substantially vertically from near to the screen toward the flat plate closing the top of the cylindrical part. In order to prevent the carrier particles escaping through the gas outlet, a suitable screen is provided near the top of the chamber. In the region underneath the screen, and underneath the horizontal plate around the screen, the hot carrier particles undergo high velocity collisions with each other, with the chamber walls and with the underside of the screen. The carrier particles then return to the bottom of the chamber in a flow near to the inside of the cylindrical and conical chamber walls. The dryer also includes at least one feed port for the slurry of material which is to be dried. The feed port, or ports, is/are often located near to the narrow end or near to the wide end of the lower conical part of the chamber.
In operation, the slurry entering the drying chamber through the feed port(s) is atomised into fine droplets which form a coating on the hot carrier particles. As the coated carrier particles move vertically upwardly in the hot gas stream essentially in the center of the chamber the coating loses any volatile liquids in the slurry, such as water, to form an essentially dry and fragile coating on the carrier particles. When the coated particles enter the region beneath the screen, in a space which may be termed the collision zone, the ensuing collisions break up and detach the fragile dried coating from the carrier particles to provide an essentially dry powder of solid material derived from the slurry. The dry powder is small enough to be carried by the hot gas flow upwardly through the screen, so that the dried solid material powder leaves the drier in the exhaust gas flow through the hot gas outlet. A suitable gas/solid separation system is used to recover the dried product from the exhaust hot gas flow.
One example of a conical jet spouted fluidised bed drier of this type in which inert particles are used as a packing of heat exchanges particles is described by Legros et al., in CA 2,178,575, and also in U.S. Pat. No. 5,809,664. In this example, the jet spouted fluidised bed drier is used to process animal manure to provide a dried product suitable for use in fertilizers.
It can be seen from the preceding description that there are two features of such a jet spouted fluidised bed drier which have a direct bearing on drier efficiency.
The first is that when the carrier particles arrive in the collision zone the coating on the carrier particles should be adequately dry, and contain more or less only the liquid level required in the dried product. If the liquid level in the dried coating is too high, adequate disengagement of the coating from the carrier particles is not possible.
The second is that recovery of the dried solid product formed as a coating on the carrier particles in the drying process relies completely on what happens to the coated carrier particles in the collision region. If the collisions involving the coated carrier particles do not result in substantially complete disengagement of the dried coating, the drying efficiency of the jet spouted fluidised bed drier is compromised, and the rate of dried coating removal does not match the rate at which the solids in the slurry are entering the drier. This can lead to a build up of solids on the carrier particles which will eventually choke the drier.
In practise it has been found that although by controlling the gas flow rate, gas temperature and slurry feed rate the required level of drying of the coating can be obtained with at least some biomaterials. But it appears that the nature of the events in the collision region in a jet spouted fluidised bed drier substantially as described by Legros et al. do not remove the dried coating efficiently from the inert carrier particles. It has also been found that it is effectively impossible to process some biomaterials in the type of drier described by Legros et al. For example, attempts to process meat rendering slurries result in the inner surfaces of at least the collision region becoming coated with oily and fatty components from the meat rendering slurry. The result of this sticky coating is that the drier becomes choked, and ceases to operate properly.
An improved jet spouted fluidised bed drier has been described by Benali et al., in Proceedings of Symposium on Energy Engineering, Hong Kong, 2000. In this improved drier, the bottom conical part of the drier chamber is retained. The top cylindrical part is replaced by an essentially hemispherical part of the same radius as the top end of the lower conical part. The gas outlet is provided at the top of the hemisphere (essentially coaxial with the lower conical part). The flat screen is replaced by a conical grid extending downwardly into the hemispherical part with the cone axis more or less coaxial with the hot gas outlet. Additionally, the slurry inlet port can be located to provide a downwardly oriented atomised slurry flow from a point below the lower end of the conical screen.
The result of these modifications is that the collision region has a quite different shape, as it is the space between the outside of the conical grid and the inside of the upper part of the hemispherical part. Although it has been found that this modified jet spouted fluidised bed drier is an improvement on the drier described by Legros et al. it still leaves considerable room for improvement. For example, it is still not capable of processing meat rendering slurry efficiently.